Two-dimensional (2D) and three-dimensional (3D) printers operate one or more ejectors to eject drops of material onto an image receiving member or platen. The material may be aqueous, oil, solvent-based, UV curable, emulsions, phase change, or other materials, particularly in three-dimensional (3D) object printers.
A typical printer uses one or more ejectors that can be organized in one or more printheads. The ejectors eject drops of material across an open gap to an image receiving member or platen. In a 2D printer, the image receiving member may be a continuous web of recording media, a series of media sheets, or the image receiving member may be a rotating surface, such as a print drum or endless belt. In a 3D printer, the platen can be a planar member on which an object is built layer by layer or a cylindrical member that rotates about the ejectors for formation of an object. Images printed on a rotating surface in a 2D printer are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. The ejectors can be implemented with piezoelectric, thermal, or acoustic actuators that generate mechanical forces that expel material drops through an orifice in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, or voltage level, of the timing signals affects the amount of material ejected in each drop. The firing signals are generated by a controller in accordance with image or object layer data. A printer forms a printed image or object layer in accordance with the image data or object layer data by printing a pattern of individual drops at particular locations on the image receiving member or previously formed layers on the platen. The locations where the drops land are sometimes called “drop locations,” “drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of drops on an image receiving member or platen in accordance with image data or object layer data.
The ejectors in 2D and 3D printers must be registered with reference to the imaging surface or platen and with the other ejectors in the printer. Registration of ejectors is a process in which the ejectors are operated to eject drops in a known pattern and then the printed image of the ejected drops is analyzed to determine the orientation of the ejectors with reference to the imaging surface or previously formed layers and with reference to the other ejectors in the printer. Operating the ejectors in a printer to eject drops in correspondence with image data or object layer data presumes that the ejectors are level with a width across the image receiving member or previously formed layers and that all of the ejectors are operational. The presumptions regarding the orientations of the ejectors, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the ejectors cannot be verified, the analysis of the printed image or layers should generate data that can be used either to adjust the operation of the ejectors so they better conform to the presumed conditions for printing or to compensate for the deviations of the ejectors from the presumed conditions.
Analysis of printed images is performed with reference to two directions. “Process direction” refers to the direction in which the image receiving member or platen is moving as the imaging surface or platen passes the ejectors to receive the ejected drops and “cross-process direction” refers to a direction that is perpendicular to the process direction in the plane of the image receiving member or platen. In order to analyze a printed image or layer, a test pattern needs to be generated so determinations can be made as to whether the ejectors operated to eject drops did, in fact, eject the drops and whether the ejected drops landed where the drops would have landed if the ejectors were oriented correctly with reference to the image receiving member or platen and the other ejectors in the printer.
Systems and methods exist for detecting drops ejected by different ejectors, inferring the positions and orientations of the ejectors, and identifying correctional data useful for moving one or more of the ejectors to achieve alignment acceptable for good registration in the printing system. The drops are ejected in a known pattern, sometimes called a test pattern, to enable one or more processors in the printing system to analyze image data of the test pattern on the drop receiving substrate for detection of the drops and determination of the ejector positions and orientation. In some printing systems, ejectors are configured to eject clear drops of material onto the receiving member or platen. This clear material is useful for adjusting gloss levels of the final printed product or the surface finish of a manufactured 3D object. Additionally, clear materials can be used to form optical structures, such as lenses on a surface of a 3D object, or to form support structures during the building of a 3D object. As used in this document, the term “clear” refers to a material that has a low or no concentration of colorant in it. One issue that arises from the use of clear material, however, is the difficulty in detecting drops of clear material ejected onto a receiving member with an imaging system. Because the clear drops do not image well, the known systems and methods for aligning ejectors do not enable the clear drops to be detected and the positions and orientations of the ejectors ejecting clear material to be inferred.
In one known system and method for aligning ejectors, the test pattern is formed with the drops ejected from the ejectors forming dashes. The dashes in the test pattern are illuminated by a light source, such as a fluorescent lamp or a light tube that extends across the width of the drop receiving member in the cross-process direction. An image sensor having a plurality of light receivers, such as photodetectors, receives the light reflected from the receiving member. As the receiving member moves past the light source and receiver in the process direction, the light is generally collimated. But in the cross-process direction, the light reflected from the image receiving member that is picked up by the light receivers can come from the whole width of the light source. This type of light leads to the edges of the dashes of clear material in the test pattern looking like the background so detecting the dashes in the image of the test pattern is difficult. This obfuscation is especially present when the clear materials are ejected on shiny or mirror-like surfaces, which are useful for detecting a wide range of colored materials and uncolored materials. Therefore, development of a system and method for aligning ejectors that can detect dashes of clear material, particularly on shiny or mirror-like substrates, in a test pattern is a desirable goal.